The US Special Operations Command(US SOCOM) issued a solicitation for the procurement of SOF CombatAssault Rifles (SCAR)on October 15th, 2003. This solicitation requested a new combat rifle,specially tailored for the current and proposed future needs of the US Special Forces,which are somewhat different from then current US Army requirements,which were being fulfilled (unsucessfully) by the newest Heckler-Koch XM8 assaultrifle. The key difference in basic requirements between XM8 and SCAR was that, while XM8 was a single-caliber weapon system, tailored for 5.56x45mm NATO ammunition, the SCAR would be available in various different calibers. Initial SOF requirements included two basic versions of SCAR system - the SCAR Light (SCAR-L), available in 5.56mm NATO, and the SCAR heavy (SCAR-H), which should be initially available in significantly more powerful 7.62x51 NATO chambering, and should be easily adaptable in the field to other chamberings (this still is not the case).The key idea of SCAR rifle system is that it will provide the Special Forces operators with wide variety of options, from short-barreled 5.56mm SCAR-L CQC variation,tailored for urban close combat, and up to long range 7.62x51 SCAR-H Sniper variant. Both SCAR-L and SCAR-H are available in three basic versions, Standard(S), Close Quarters Combat (CQC) and Long Barrel (LB). All these variants, regardless the caliber and exact configuration, willprovide the operator with the same controls layout, same handling and maintenance procedures, and same optional equipment, such as sights,scopes, andother current and future attachments. Since 2014, an even shorter version of the SCAR-L is available as SCAR-L PDW - Personal Defense Weapon.

Late in 2004 USSOCOM announced, that the winner for the initial SCAR contracts is the FN USA, an US-based subsidiary of the famous Belgian company Fabrique Nationale Herstal. Зrototype rifles were manufactured by FN Manufacturing Inc, US-based subsidiary to FN Herstal; This company will also handle series production of rifles. Starting mid-2005, first SCAR rifles went to end users in US Special Operation Forces. Since USSOCOM uses Navy-type "mark" designations, SCAR rifles were officially designated as 5.56mm Rifle Mark 16 (SCAR-L / Light) and 7.62mm Rifle Mark 17 (SCAR-H/ Heavy). Despite original plans, only 7,62mm SCAR-H rifles are in current service with Us Special Forces. However, SCAR system enjoys steady and growing sales worrldwide, with numerous Special Forces buying both 5.56mm and 7.62mm versions. Finally, it seems that Belgian army is adopting 5.56mm SCAR-L as a general issue infantry rifle, to replace aging 5.56mm FN FNC rifles which are no longer made by FN.

All variants of FN SCAR rifles feature gas operated,short stroke piston action with rotating bolt locking. Bolt has seven radial locking lugs that lock directly into the barrel extension.
Receiver is made from two parts, upper and lower, connected with two cross-pins. Upper part is made from extruded aluminium, lower part is made from polymer. SCAR-L and SCAR-H use similar upper receivers that differ only in the size of ejection port. Other different parts include caliber-specific bolts, barrels, and lower receivers with integral magazine housing. Parts commonality between SCAR-L and SCAR-H is an astonishing 90%. Barrels are quick-detachable, and held in the upper receiver with two cross-bolts. Barrel change procedure requires minimum amount of tools, takes just several minutes and there is no need to adjust the headspace after the change.
The trigger unit with ambidextrous safety-fire mode selectors witch allows for single shots and full automatic fire, with no provisions for limited-length bursts mode. The charging handle could be easily installed on either side of the weapon, so the upper receiver has respective cuts on both sides. Top of the upper receiver is covered by the full-length integral Picatinny rail (MIL-STD 1913); additional Picatinny rails are mounted on both sides and under the free-floating handguards. Side-folding polymer buttstock is adjustable for length of pull, and is shaped to provide positive cheekrest with adjustable cheek support. SCAR rifles are fitted with removable, adjustable iron sights, with folding diopter-type rear sight on the receiver rail, and folding frontsight onthe gas block. Any additional type of sighting equipment, necessary for current tasks, including telescope and night sights, can be installed using MIL-STD 1913 compatible mounts.
Mk.16 SCAR-L rifle uses any 5.56mm STANAG (M16-type) magazines; Mk.17 SCAR-H uses proprietary 20-round magazines in 7.62x51 NATO.

Special thanks to Charles Cutshawfor invaluable information and images

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